When selecting the app for your relationship field, use the "Pick a view" button on the right (pictured below) to only allow specific items to be tagged in this field. This lets you pick from any of your saved views.

Now when you add an item to your app, from the Relationship field you can select items from your related app. If the related item doesn't exist yet, you can create a new item from here as well - select "Create new item" at the bottom to do so.